Signaling system



June 9, 1936. D, MITCHELL SIGNALING SYSTEM Filed Sept. 25, 1935 3mm sINVENTOR J), Mitchell ATTORNEY Patented June 9, 1936 UNITED STATESPATENT OFFICE SIGNALING SYSTEM Doren Mitchell, Bound Brook, N. 1.,assignor to American Telephone and Telegraph Company, a corporation ofNew York Application September 25, 1935, Serial No. 42,088

5 Glaims.

' ing device now known in the art. as. a vogad,

A vogad is a device which receives speech messages of dlfierent volumesas, that from a strong speaker and that from a weak speaker, and withincertain limits changes or adjusts its own gain in such manner that thevolume delivered by the vogad is practically constant, that is,practically independent of the volume of the incoming speech. Such adevice may take on a variety of forms, but its general structure andbehavior is well indicated by the patent to Hogg et al., No.

1,853,974, April .12, 1932. It consists essentially of a gain devicesuch as a vacuum tube amplifier, the gain of which is determined by thebiasing voltage on its grid. This biasing voltage, usually determined bythe charge on a condenser, is increased or decreased, or heldstationary, depending on the volume of the'incoming speech message.

The vogad, in general, has, a gain increaser associated with theinput ofthe amplifier. which permits change of charge on a condenser if thereceived volume is below a certain value in such direction as to,increase the gain of the amplifier. It also has a gain decreaserassociated with the output of the amplifier whichchanges the charge onthe condenser in. the reverse direction if the output of the amplifieris too. large. And, it also has a gain increase disabler which preventsfurther effect by the gainincreaser if the output volume is above acertain level. All of this is described in the. above patent to. Hoggeta1., and its association, as well as its cooperation, witha telephonecommunication system is further disclosed, quite fully, in BritishPatent, No. 381,831.

A transmission line'having the volume of the signals thereon maintainedsubstantially constant by such a vogad will, at the time of connecting atoll line or subscriber line to. the transmission line, have the vogadfirst adjusted by the, speech of the operator at the central oilicemaking. the connection, and then by the speech of the sub.- scriber onthe toll line. The speech currents, of the operator will usually berelatively strong and operate the vogad circuits to lower the gain onthe transmission line prior to the operation of the vogad by the speechcurrents-of the .subscriben. The voice currents. of. the subscriberreceived-on the. transmission linefroni the toll line. are often muchweaker than.- the. voice currents received on the transmission. line.from. the

operators talking circuit. Accordingly, at the time of connection of thetoll line to the transmission line, the voice operated gain controlcircuits are often set for a much lower gain than is I required by thespeech currents from the. toll line, and this may, in fact, be so lowthat if the subscriber is a weak'speaker his speech message may notefiectively operate the various devices which it is expected to operate,until the lapse of an appreciable time during which several syllables ofthe speech. may be lost.

The object of this invention is to provide means whereby this difiicultyis obviated. More specifically, one object of the invention is toprovide a central ofiice for connecting a toll. line to a transmissionline having a vogad connected thereto, with means under the control ofthe operator at the central office to adjust the setting of the vogadapproximately to the appropriate setting for a talker on the line and todo this prior to arrival of speech message from the toll line. Anotherobject of the invention is tov do this without resorting to the use ofdirect current supply through the cord operators circuit. This for thereason that most of the D. C.v channels, I

both on the sleeve and on the main transmission circuit, will alreadyhave been set aside for other functions.

The invention will be better understood by ref.- erence to the followingspecification andv the. accompanying drawing, in which Figure 1 showsone specific circuit, arrangement by which my invention may be carriedout, and Fig. 2 shows a modification ofv a portion of the circuit ofFig. 1 and indicates a different. method by which the broad purposesofthe invention, as given above, may be. carried out.

Broadly, the circuit, which is shown inv the figures to remedy the.difiiculty, provides a function in the operation of. the system sothat. when the operator throws her talking key from the talk to. thenormal position ,(or from talk to monitor) she will cause the vogad tobe brought to a neutral condition, and also under the same control willcause its gain to be brought up to some predetermined value. This.action will take place in a very short time, and after that the actionof the. vogad will proceed. in a normal manner.

Referring particularly to Fig. 1,, which illustrates. one circuit forcarrying out.v my invention, there is shown atv the left-hand side ofthe drawing. the operators cord a which is adapted for associating theline L1 with the transmission line L2, the whole being adapted. tochange the. communication circuit from a. two-wire basis,.as in.

L1, to a four-wire basis, as in L2. Associated with the transmissionside H of the transmission line L2, is a source of oscillations l2preferably of a frequency somewhat above that to be used for thetransmission of speech. This generator is supplied primarily to performa number of functions at the terminal of the line L2, some of whichfunctions are described in detail in the British patent, cited above, orin the application of Wright & Mitchell, Serial No. 756,334 filedDecember 6, 1934, and in this modification of my invention I propose touse this source of oscillations to carry out the purpose of thisinvention. The transfer from the two-wire to the four-wire basis, or thereverse, is accomplished by means of a hybrid terminating set E3 of aform now well known in the art. With this hybrid coil is associated abalancing network I4 of such impedance characteristics as to quiteclosely match the impedance of the incoming toll line over the band offrequencies used for speech transmission. Under these circumstances, asis well understood, the transmitting side of the line H is in conjugaterelationship to the receiving side I5. In the main transmission path ofthe operators cord circuit I include a 4,000 cycle low-pass filter whichwill cause practically no loss in the voice range (250 to 3,000 cycles)but will present a high impedance at 6,000 cycles. Also, bridged acrossthe cord circuit is a circuit consisting of a resistance I8 of say 600ohms in series with a 6,000 cycle tuned circuit this circuit beingclosed when the operators key is thrown. The introduction of this tunedshunt circuit will cause very little reaction on the voice band, butwill effectively terminate the circuit in approximately 600 ohms at afrequency of 6,000 cycles. The value of 600 ohms is taken forillustrative purposes on the basis that the impedance, as seen lookinginto the toll line, is 600 ohms, this being a more or less standardvalue in present day telephone communication. If the impedance of theline should change, then the impedance of the balancing network and theimpedance of the shunt circuit should be correspondingly changed.

Referring more specifically now to the fourwire terminating circuit,there is shown the vogad and the arrangements for bringing it to aneutral condition. For simplicity the vogad is shown in conventional.form only, and briefly, it consists of the balanced amplifier circuit20, the gain of which is determined by the biasing voltage of thebattery 2| and the charge on the condenser 22. Associated with thisamplifier are the gain increaser 23, the gain decreaser 24, and the gainincrease disabler 25. As described in greater detail in the referencesmentioned above, the gain increaser 23 operates through the relay 3| topermit the leakage of charge from the condenser 22, this relay beingoperated whenever the incoming speech is above that corresponding to theweakest speech to be transmitted. If the incoming speech is above thisminimum level and if the gain of the amplifier 20 is such as to give apower at the output of 20 in excess of a predetermined amount, the gainincrease disabler 25 operates the relay 32 to open the conductor 34,thus preventing further leakage of charge from the condenser 22 throughthe contact of relay 3|. Furthermore, if the power output of theamplifier 20 exceeds a certain predetermined value the gain decreaser 24permits the building up of a charge on the condenser 22 in a directionto place further negative bias on the grids of the amplifier 20, thusdecreasing the gain.

In order to control the rate at which the corn denser 22 discharges onoperation of the relay 3|, a resistance 26 is introduced in series withthe conductor 34. Also, in order to give the disabler 25 a desired slowrelease characteristic a condenser 38 and a resistance 39 are insertedin the output circuit of the disabler 25, in the manner shown. As thusfar described, the circuit has the characteristics of the vogads used inthe circuits referred to above.

Certain additions to the circuit, in order to carry out the purposes ofmy invention, will now be described. When the operators talking key isnormal the balance between the network on the four-wire terminating setand the line is very poor at 6,000 cycles due to the fact that, althoughthere is a 600 ohm impedance in the balancing network, the low passfilter in the operator's set offers a high impedance at 6,000 cycles.Consequently, 6,000 cycle power will be transferred across the hybridcoil and pass through a bandpass filter 4| to operate theamplifier-detector 42 with its relay 44. This relay in turn operates twoother relays, one of which, 46 is quick operating, and the other ofwhich, 41, is slow operating. The quick operating relay 46 appliesbattery to the armature of the slow operating relay 47, but the latter,upon being operated, removes battery from a restoring relay 49. Sincethe latter relay is unoperated the action of the vogad is normal, asdescribed above.

When the operator throws the talking key, closing contact at iteffectively terminates the line with 600 ohms, so far as 6,000 cycles isconcerned, and the lines I and I5 are then substantially conjugate toeach other so that there is a very considerable decrease in the 6,000cycle power passing through the hybrid coil to the filter 4|.Consequently, the relay 44 is released and this causes both the quickoperating and .slow operating relays 45 and 47 to release, thus removingbattery from the contact of the relay 46 and from the armature of therelay 47.

During the time that the operator talks the gain of the vogad will beadjusted to a fairly low value due to the operators loud speech. Whenshe finishes talking, however, and releases the talking key, the 6,000cycle power is again transmitted with comparatively small loss acrossthe hybrid coil thus operating the relay 44. This causes the almostimmediate operation of the quick operating relay 46 thus placing batteryon the armature of the slow operating relay 47. During the time that thearmature of the relay 4! does move, however, the restoring relay 49 willbe operated, and this latter performs two functions.

In the first place, a substantial portion of the resistance 39 isshort-circuited, thus the condenser 38 which gives the relay 32 its slowrelease is charged up very quickly and this relay has to release placingthe device, as a whole, in a condition to increase gain if necessary. Asmall portion of the resistance 39 is still preferably kept in thecircuit so that the tube in the gain increase disabler 25 will not beharmed by an excessive value of current.

The second function performed by the restoring relay, when thusoperated, is to connect a battery 53 across the condenser 22 whichcontrols the gain of the variablerepeater or amplifier 20. This battery53 may be of any specified value, positive or negative or zero, whichwill give the repeater 20 such predetermined value or gain as isdesired.

The operations described above can be made to take place in acomparatively short time, perhaps about 1/20 of a second. After that theslow operating relay 41 will operate, the restoring relay 49 willrelease, and the operation of the vogad will be normal. However, it willbe evident that the disabling of the slow release of the relay 32 andthe setting of the gain of the repeater 20 at some fairly highpredetermined value will give the subscriber a better opportunity toboth increase the gain of the vogad, if necessary, and successfullyoperate the other apparatus incidental to the circuit, as a whole, onhis first few Words.

In the circuit of Fig. 1, use has been made of the oscillator l2 for thereason that in some circuits of this type such a generator is present inconnection with certain other functions, which are to be performed. Itshould be apparent, however, that an oscillation generator, quiteindependent of the generator l2 and even of a different frequency, maybe used if .desired; that such a generator might be supplied for thespecific purpose of this invention in the event that an oscillationgenerator is not already present in connection with the performance ofother functions, and also that such a generator might be placed in otherportions of the circuit. Such a modification is shown, for example, inFig. 2, in which generator 60 of any desire-d frequency, preferablyoutside the voice range, is placed in series in the tuned circuit H, thecontact to be controlled by the operators key being such, as shown inFig. 2, that the generator circuit is normally closed permitting theflow of power from the generator to the filter 4| in the same mannerthat power ordinarily flows from the generator 12 of Fig. 1. When theoperator talks the contact at 63 is opened and the relay 44 is released.When she finishes talking, however, power from the generator 60 is againtransmitted and the relay 44 is operated to bring about the succeedingsteps, all as described in connection with Fig. 1.

What is claimed is:

1. In a signaling system, a transmission path, control means forautomatically maintaining the signals on said path at substantiallyconstant volume, a toll line, a central ofiice having means forconnecting said toll line to the transmission path, an A. C. sourceassociated with the transmission path, and means operated at the centralofiice at the time of connecting the toll line to the transmission pathand prior to the operation of the control means by the signals from thetoll line whereby power from the A. C. source resets the control meansto a value appropriate for the volume of signal coming from the tollline.

2. In a signaling system, a transmission path, a vogad associated withsaid path for maintaining the signals on the path at substantiallyconstant volume, an A. C. source of a definite frequency associated withthe transmission path, a toll line, a central oflice having means forconnecting said toll line to the transmission path, and means associatedWith the vogad for quickly increasing its gain, said means beingcontrolled by current from the A. C. source, and further means forpermitting the said current to exercise its control at the time ofmaking the connection to the toll line.

3. In a circuit of constant volume having separate paths fortransmitting in opposite directions and adapted to be switched to atwo-Wire subscriber circuit, a vogad in the transmission path of thefirst-mentioned circuit to deliver substantially constant volume ofspeech message, a source of pilot current associated with thetransmission path, an operators set with a cord circuit at the switchingpoint delivering relatively high volume to the input of the vogad and soreducing its gain, means associated with the vogad and controlled bycurrent from the pilot source which upon its connection to the operatorstalking key quickly resets the vogad gain to a normal gain.

4. In a circuit of constant volume having separate paths fortransmitting in opposite directions and adapted to be switched to atwo-wire subscriber circuit, a vogad in the transmission path of thefirst-mentioned circuit to deliver substantially constant volume ofspeech message, a source of pilot current associated with thetransmission path, an operators set with a cord circuit at the switchingpoint delivering relatively high volume to the input of the vogad and soreducing its gain, means associated with the vogad and controlled bycurrent from the pilot source which upon its connection to the operatorstalking key quickly resets the vogad gain to a gain appropriate for adistant subscribers message.

5. In a circuit of constant volume having separate paths fortransmitting in opposite directions and adapted to be switched to atwo-wire subscriber circuit, a vogad in the transmitting path of thefirst-mentioned circuit to deliver substantially constant volume ofspeech message, an operators set with a cord circuit at the switchingpoint, the operators set delivering relatively high volume to the inputof the vogad and so reducing its gain, an oscillation generatorassociated with the cord circuit and controlled by the key of theoperators set, and means associated with the vogad whereby upon thisconnection of the operators talking key power from the generator quicklyresets the vogad gain to a gain appropriate for the distant subscribersmessage.

DOREN MITCHELL.

